This invention relates to the determination of the optimum isothermal heat treatment procedure for alloys such as steel and austempered ductile iron. For years, workers have followed various procedures to determine the transformation characterstics and proper heat treatment procedures to obtain the optimum properties for a given alloy.
There are many problems associated with the various methods employed for these purposes in the past. The original method, which is still used, was a trial and error method. Another method presently used includes the steps of preparing a number of specimens, austenitizing them at appropriate temperatures and then cooling them rapidly by immersion in a liquid maintained at a lower temperature where the transformation of austenite into the desired microstructure (e.g. bainite, martensite) occurs. The specimens are then removed and examined to determine the microstructure.
An improved method uses metallurgical "quench" dilatometers which use a hollow specimen that can be quickly cooled from austenitizing temperatures under controlled conditions by a controlled combination of R.F. heating and gas cooling. The progress of the transformation in this type of system is monitored dilatometrically (i.e. by measuring length changes that accompany the phase transformation).
The metallurgical dilatometer is an excellent tool however it has the disadvantage of using relatively complex instrumentation with associated maintenance difficulties. In addition, false results are often obtained.